Why has Nintendo not put up GBA titles on the eShop?

This topic has probably come up before but I browse this subreddit less than frequently. I'm not talking about the ambassador program, I was a part of it and have my free games. Knowing that Nintendo CAN put the game on the 3DS make me wonder why they don't. Recently I purchased a GBA SP (can't stand my GBA anymore since they don't have back lights) and 3 GBA games on eBay, totaling around $75. Money which I would have gladly gave to Nintendo for the games on the 3DS.